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Materials Chemistry and Physics 119 (2010) 465–470

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

tudies in structural characterization of silica–heteropolyacids compositesrepared by sol–gel method

lexandru Popaa,∗, Viorel Sascaa, Erne E. Kissb, Radmila Marinkovic-Neducinb,ilos T. Bokorovc, Ivanka Holclajtner-Antunovicd

Institute of Chemistry Timisoara, Bl. Mihai Viteazul 24, 300223 Timisoara, RomaniaUniversity of Novi Sad, Faculty of Technology, Cara Lazara 1, Novi Sad, SerbiaUniversity Centre For Electron Microscopy, University of Novi Sad, Dositeja Obradovica 2, SerbiaUniversity of Belgrade, Faculty of Physical Chemistry, P.O. Box 47, 11158 Belgrade, Serbia

r t i c l e i n f o

rticle history:eceived 24 March 2009eceived in revised form0 September 2009ccepted 24 September 2009

a b s t r a c t

Heteropolyacids (HPAs) which are included in mesoporous silica, H3PMo12O40/SiO2 andH4PMo11VO40/SiO2 have been synthesized by a sol–gel technique that involves hydrolysis of ethylorthosilicate. The effect of incorporation of heteropolyacids species on silica matrix was studied bypowder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and Raman spectroscopy, thermogravimetric analysis (TGA) and differential thermal analysis (DTA), N2 adsorption–desorption, scanningelectron microscopy (SEM) and energy dispersive spectroscopy (EDS).

eywords:eteropolyacidsanocompositesilicaol–gel

X-ray powder patterns prove that heteropolyacids are uniformly dispersed in the silica network. IRand Raman spectra have show that the HPAs anions preserved their Keggin structure on the surface ofsilica–HPA composites. SEM images and BET adsorption–desorption isotherms confirm the presence ofnanometer particles and mesoporous structures. SEM micrographs of silica–HPAs composites show thatsilica–HPAs composites are composed of spherical particles with an average diameter of approximately

uded

20–30 �m. The silica-incl

. Introduction

In the last two or three decades heteropolyacids (HPAs) haveeen studied extensively for the molecular design of mixed oxidesatalysts and for other practical uses as compounds with pho-oconductive and magnetic characteristics, reagents in analyticalhemistry, super ionic proton conductors and biochemically activepecies [1]. HPAs are a class of materials with well-defined struc-ures and tuneable acidity, characteristics that make them valuableatalysts for both acid–base and redox reactions. Heteropolyacidsould have different polyanion structures and molecular arrange-ents like Anderson, Dawson and Keggin structures, but most of

he attention of researchers has been focused on the latter type [2].HPAs with Keggin structure have been applied to catalyze a large

pectrum of chemical reactions ranging from selective oxidation

rocesses to acid-catalyzed transformation of organic molecules,oth in the heterogeneous and homogeneous systems. The advan-ages afforded by the use of Keggin-type HPAs include strongcidities, lower proportion of side reactions and no toxic waste.

∗ Corresponding author. Tel.: +40 256 491818; fax: +40 256 491824.E-mail address: alpopa tim2003@yahoo.com (A. Popa).

254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2009.09.026

heteropolyacids were thermally more stable than the parent ones.© 2009 Elsevier B.V. All rights reserved.

As pure HPAs generally show low catalytic reactivity owing totheir small surface area, they are usually impregnated on differentporous materials with high surface area (silica, titania, polymers,molecular sieves) [3–6] or included on a silica matrix by sol–gelmethods [7–18]. Izumi et al. have prepared silica-included HPAscatalysts such as H3[PW12O40]/SiO2 and H4[SiW12O40]/SiO2 byadding H3[PW12O40] and H4[SiW12O40] during the hydrolysis oftetraethyl orthosilicate (TEOS) [7–9].

In the selective oxidation of unsaturated aldehydes and oflow alcohols most frequently used are molybdophosphoric acidH3[PMo12O40]·xH2O (HPM) and 1-vanado-11-molybdophosphoricacid H4[PMo11VO40]·yH2O (HPVM) in consequence of their highcatalytic activity. In order to obtain highly dispersed heteropoly-acids species, HPM and HPVM were included in a silica matrixby sol–gel methods. The goal of this work was to characteriseextensively the texture and the structure of these heteropolyacidsincluded in silica matrix in reference to the bulk solid heteropoly-acids.

2. Materials and methods

2.1. Samples preparation

H4[PMo11VO40]·12H2O was prepared by two methods: Tsigdinos andhydrothermal methods [10,11]. In both cases HPAs were crystallized slowly from

4 stry and Physics 119 (2010) 465–470

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ı PO4, respectively [13,14]. These bands are preserved on the sup-ported samples, but they are broadened and partially overlappedbecause of the strong absorption bands of silica (1090, 940, 800 and465 cm−1) (Fig. 2).

66 A. Popa et al. / Materials Chemi

queous solutions at room temperature. H3[PMo12O40]·13H2O was purchased fromerck.

Silica-included HPAs (denoted HPA-in-SiO2) with 11% HPA loading (weightatio silica/HPA was 8) were prepared by the hydrolysis of tetraethyl orthosilicate.he procedure described by Molnar et al. [12] was applied with some modi-cations. Tetraethyl orthosilicate (TEOS, 47.91 g) was dissolved in a mixture of8.56 g ethanol and 20 g water while stirring at 40 ◦C. To this solution heteropoly-cids (HPM or HPVM) were added in calculated amounts to reach 11 wt.% activehase loading in the final (calcined) silica–HPA composites. The hydrolysis ofEtO)4Si was carried out at the following ratios H2O/(EtO)4Si = 4.8 mol mol−1 andtOH/(EtO)4Si = 2.7 mol mol−1. After stirring for 1 h at 40 ◦C and at 80 ◦C for 4 h aydrogel was obtained. The resulted hydrogel was aged at ambient conditions for4 h and then dried at 80 ◦C for 4 h. The final materials were a transparent yellowHPM-in-SiO2) stable gel and a transparent orange (HPVM-in-SiO2) gel, respectively.

The structure and texture of HPM and HPVM included on silica were stud-ed by XRD, FT-IR and Raman spectroscopy, low temperature nitrogen adsorptionechnique and scanning electron microscopy with EDS analysis.

.2. Measurements of textural properties

Textural characteristics of the outgassed samples were obtained from nitrogenhysisorption using a Quantachrome instrument, Nova 2000 series. The specific sur-ace area SBET, mean cylindrical pore diameters dp and adsorption pore volume VpN2ere determined. Prior to the measurements the samples were evacuated under

acuum, at 250 ◦C. The BET specific surface area was calculated by using the stan-ard Brunauer, Emmett and Teller method on the basis of the adsorption data alonghe partial pressure P/P0 range from 0.01 to 0.3 and taking a cross-sectional areaf 0.162 nm2 per nitrogen molecule. The pore size distributions were calculatedpplying the Barrett–Joyner–Halenda (BJH) method to the desorption branches ofhe isotherms. The IUPAC classification of pores and isotherms was used in thistudy.

.3. XRD analysis

Powder X-ray diffraction data were obtained with a XD 8 Advanced Brukeriffractometer using the Cu K� radiation. The scanning was made for a 2� anglerom 5◦ to 60◦ , with a step size of 0.02 and a step time of 2 s.

.4. Surface characterization by Fourier transform infrared (FT-IR) spectroscopynd Raman spectroscopy

The surface dehydroxylation of the dried samples was characterized by the FT-R spectroscopic technique on a Jasco 430 spectrometer. The IR absorption spectra

ere recorded in a spectral range 4000–400 cm−1, with 256 scans and a resolutionf 2 cm−1, using KBr pellets.

Raman spectroscopic investigations were carried out using a DXR Raman micro-cope (Thermo Scientific) equipped with DXR 532 nm Excitation Laser Set at powerf 10 mW. Spectra were collected at room temperature in the wavelength range–3500 cm−1 at a spectral resolution of 5 cm−1.

.5. Thermal analysis

Thermal decomposition was carried out using a TGA/SDTA 851-LF 1100 Mettlerpparatus. The samples with mass of about 300 mg were placed in alumina cruciblef 900 ml. The experiments were conducted in air or nitrogen flow of 50 ml min−1,n the temperature range of 25–600 ◦C with a heating rate of 10 ◦C min−1. The airupplied by a compressor (4–5 bar) was passed over granular silica gel. The nitrogenas supplied from Linde gas cylinder (150 bar) of 4.6 purity class.

.6. Scanning electronic microscopy (SEM) and EDS analysis

Microstructure characterization of the catalyst particles was carried out with aEOL JSM 6460 LV instrument equipped with an Oxford Instruments EDS analyser.owder materials were deposited on adhesive tape fixed to specimen tabs and thenon sputter coated with gold.

. Results and discussion

The XRD patterns of silica–HPA composites prepared by sol–gelrocedure and spectra of pure HPAs are shown in Fig. 1. The most

ntensive reflections of bulk HPM and HPVM appear in the follow-ng 2� intervals: 7–10◦, 17–23◦ and 26–30◦. For both HPAs–silica

omposites could be observed that a large and flat diffraction peakppeared at 2� = 15–35◦ attributed to amorphous silica. Besidehe large and flat diffraction peak, another large diffraction peakppeared between 2� = 5–10◦, but only in the HPA–silica compos-tes (Fig. 1). This indicates that active phases of HPAs are probably

Fig. 1. X-ray diffraction pattern of pure heteropolyacids and ofsilica–heteropolyacids composites.

presented in very small clusters undetectable by XRD as sharp andnarrow diffraction peaks.

In order to confirm the presence of the Keggin anion onsilica–HPA composites, the samples were analysed by FTIR. ThePMo12O40

3− Keggin ion structure consists of a PO4 tetrahedronsurrounded by four Mo3O13 formed by edge-sharing octahedra.These groups are connected each other by corner-sharing oxygen.This structure give rise to four types of oxygen, being responsi-ble for the fingerprints bands of Keggin ion between 1200 and700 cm−1.

The pure HPAs show IR spectra with the specific bands of theKeggin structure (Fig. 2) containing the main vibrations at 1064,965, 864, 785 cm−1 assigned to the stretching vibrations �as P–O, �as

Mo Ot, �as Mo–Oc–Mo and �as Mo–Oe–Mo. The adsorption bandsat 595 and 505 cm−1 are due to the bending vibration: ı P–O and

Fig. 2. FTIR spectra of pure heteropolyacids and of silica–heteropolyacids compos-ites.

stry and Physics 119 (2010) 465–470 467

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Table 1Textural properties of silica and silica–heteropolyacids composites.

Sample Surface area(m2 g−1)

Average porediameter BJHDes (Å)

Pore volumeBJHDes (cm3 g−1)

after HPAs incorporation. For example, the BET surface area andthe pore volume of silica (538 m2 g−1, 0.39 cm3 g−1) decrease afterHPAs incorporation to 466.5 and 430.4 m2 g−1 and to 0.35 and0.22 cm3 g−1 for HPM and HPVM, respectively.

A. Popa et al. / Materials Chemi

The introduction of heteropolyacids into the silica matrixlightly influenced the structure of resulted composites (Fig. 2). Theibration band at ca. 1090 cm−1 can be assigned to �as(Si–O–Si) andecreased to 1072 cm−1 by incorporation of HPAs into the structuref the silica. The band at ca. 940 cm−1 present in the spectrum ofilica sample can be assigned to the Si–O stretching vibration ofi–O−R+ group (R+ H+). The bands at 800 and 467 cm−1 can bessigned to �s (Si–O–Si) and ı (Si–O–Si) bonds, respectively [19].

The bands of HPAs included on silica in the 1300–400 cm−1

egion are partially or completely overlapped by the bands of theilica matrix (Fig. 2). The band assigned to the P–O asymmetrictretching vibration at 1064 cm−1 is completely overlapped by thetrong band at 1090 cm−1 of the silica. Two strong bands in thepectra of included HPM and HPVM, which are shifted, appearedt 952 and 797 cm−1, as a result of the overlapping of the absorp-ion bands of silica at 940 and 800 cm−1 and those of pure HPAs at65 and 785 cm−1, respectively. Another shifted band with moder-te intensity was observed at 566 cm−1 which corresponds to theending vibration ı P–O at 595 cm−1 in pure HPAs spectra.

The characteristic bands observed in the low wave numberange of Raman spectra for pure HPM heteropolyacid are caused byeformational vibrations both of the terminal Mo O groups and ofhe entire framework. The other characteristic bands of the spec-rum in the range of 900–1100 cm−1 are attributed to the valenceibrations of the individual M O groups, the PO4 and the pulsationibrations of all 12 Mo O groups.

The Raman spectrum for bulk HPM gives the main bands at 996,83, 882, 603, and 246, 154 and 109 cm−1 which are assigned toymmetric (�s) and asymmetric (�as) vibrations of terminal oxygens (Mo–Ot) and �as (Mo–Ot), of corner shared bridged oxygen �s

Mo–Ob–Mo), of edge shared bridged oxygen �s (Mo–Oc–Mo) andf oxygen in the central tetrahedron �s (P–Oa) [20,21].

Raman spectra of pure silica, HPM, silica-included HPM, HPVMnd silica-included HPVM are shown in Fig. 3. In the Raman spec-ra of HPM and HPVM included on silica, the presence of the

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trongest characteristic bands �s(Mo–Ot) at 994 cm , �s(Mo–Oa)t 245 cm−1 and ı (Mo–O–Mo) at 151 and 100 cm−1 confirm theresence of Keggin anion included on SiO2.

The textural properties of the solids were measured by the N2dsorption–desorption isotherms method. While the BET surface

ig. 3. Raman spectra of pure heteropolyacids and of silica–heteropolyacids com-osites.

SiO2 sol–gel 538.0 21.5 0.39HPM-in-SiO2 466.5 16.4 0.35HPVM-in-SiO2 430.4 16.3 0.22

area of pure heteropolyacids was below 5 m2 g−1 [4], incorpo-ration of HPAs into the silica matrix gave a higher surface area(Table 1). Fig. 4a shows the curves of the N2 adsorption–desorptionisotherms of the silica and HPM or HPVM included on silica. Allstudied samples show a typical adsorption curve of type IV. For sil-ica is evidenced an obvious hysteresis loop at a relative pressure ofp/p0 = 0.4–0.8, while for silica–HPA composites a narrow hysteresisloop is observed.

The textural properties (BET surface area, pore volume and aver-age pore size) of silica and silica–heteropolyacids composites areshown in Table 1. All textural properties values of silica decrease

Fig. 4. (a) Nitrogen adsorption–desorption isotherms of SiO2 (a), HPM-in-SiO2 (b)and HPVM-in-SiO2 (c) at 77 K. (b) Pore size distribution of SiO2 (a), HPM-in-SiO2 (b)and HPVM-in-SiO2 (c).

468 A. Popa et al. / Materials Chemistry and Physics 119 (2010) 465–470

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Fig. 6. TG-DTG and DTA curves of silica–HPM heteropolyacid composite.

Electron microscopic studies were performed for silica–HPAscomposites using SEM mode. The micrographs of silica andsilica–heteropolyacids composites registered at low magnifica-tion (5000×) are displayed in Fig. 8. The SEM image shows that

Fig. 5. TG-DTG and DTA curves of bulk HPM heteropolyacid.

The pore size distribution curves of silica and HPM or HPVMncluded on silica have narrow pore size distribution within meso-ore and micropore range with a maximum at 21 and 16 Å,espectively (Fig. 4b). It could be observed that after HPAs incor-oration in silica matrix, the average pore diameter of silica–HPAsomposites decreased from 21.5 Å in the case of pure silica to6.4 Å for HPM-in-SiO2 and to 16.3 Å for HPVM-in-SiO2, respec-ively.

During the thermal treatment of pure heteropolyacids HPMnd HPVM the main processes are: the hydrated (or crystalli-ation) water elimination in several steps, the decomposition ofhe anhydrous heteropolyacids by constitutive water removal (allccompanied by endothermic effects) and, finally, the crystallisa-ion process of constitutive oxides of Mo and V accompanied byxothermic effects [22,23].

In the region of the hydrated water elimination, the DTA curveFig. 5) of bulk HPM shows three endothermic peaks at 72, 105nd 116 ◦C, which may be assigned to bonded water from the crys-al hydrates with different numbers of water molecules [22]. FromG and DTG curves it could be observed the loss of water of crys-allisation processes accompanied by a considerable weight loss.n the temperature range from 150 to 350 ◦C for HPM and from50 to 300 ◦C for HPVM, no weight changes or thermal effects werebserved. The final process evidenced by an exothermic peak at30 ◦C is assigned to the crystallisation of constitutive oxides: MoO3nd V2O5.

In the case of both heteropolyacids HPM and HPVM includedn silica from TG, DTG and DTA thermal curves, one can see aifferent behaviour in comparison with pure HPAs in the tem-erature range corresponding to the elimination of the hydratedater (Figs. 6 and 7). The first endothermic effect (120 ◦C) is due

o the additive thermal effects of the desorbed water from silicaurface and to the loss of the first part of the HPA crystallisationater. The second endothermic effect appears at 195 ◦C and is due

o the loss of the second part of the crystallisation water. The lossf the hydrated water is completed at 260–270 ◦C i.e. at tempera-ures which overrun those of the unsupported HPA with 80–100 ◦C,hich is due to the increased hydrophilic features of the SiO2

upport.The lack of a clear delimitation between the processes of water

elease – as it were a continuous loss of hydrated and constitutionalater – is probably due to porous texture of silica, which could

ause a delay (or even a blocking) of water elimination. In the tem-erature range 400–600 ◦C a slow and continuous loss of sampleeight are proceeding but not any peaks on DTA curve could be

Fig. 7. TG-DTG and DTA curves of silica–HPVM heteropolyacid composite.

observed. Therefore immobilization on silica obviously increasesthe thermal stability of the Keggin structures in comparison withtheir pure bulk heteropolyacids.

Fig. 8. SEM micrographs of HPM in SiO2.

A. Popa et al. / Materials Chemistry and Physics 119 (2010) 465–470 469

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ig. 9. (a and b) Microanalytical data of a 10 �m × 10 �m area and quantitativeesults of HPM in SiO2.

ilica–HPAs composites are composed of spherical particles withn average diameter of approximately 20–30 �m. On the silica par-icles surface can be observed very fine sub-particles of irregularhape which are probably responsible for high specific surface areaf the support. The surface morphology of silica–heteropolyacidsomposites is practically identical to that of the pure silica. FromEM images one can see that no separate crystallites of the bulkhase of HPAs were found in the composites.

HPAs distribution on samples surface was analysed by EDSethod, which was performed as point analysis on thin parti-

les. By this technique were obtained the chemical compositionf silicon from silica, and Mo, V and P elements of heteropolyacidrom silica–heteropolyacids composites. The EDS point analysisas acquired over several domains with 10 �m × 10 �m dimen-

ions, or less, on the same particle. The analysis was repeated onifferent particles of the same batch in order to ensure the repro-ucibility of the obtained results.

Microanalytical data of EDS analysis show that the molybde-um and phosphorous (11 wt.% HPM in SiO2) and molybdenum,hosphorous and vanadium (11 wt.% HPVM in SiO2) content isomogeneous and close to stoichiometric values.

In the case of silica-included HPM the content of Mo as % wt.s 7.78 (stoichiometric value is 6.94), while P content could note detected (stoichiometric value is 0.18 wt.%) (Fig. 9a). For silica-

ncluded HPVM the content of Mo as % wt. is 6.2 (stoichiometricalue 6.51), P content could not be detected (stoichiometric values 0.19) and V content is 0.28 (stoichiometric value is 0.31) (Fig. 10a).

In order to check the correctness of the values for Mo andcomposition, the composition values of the support elements,

espectively, silicon and oxygen were analysed.EDS analysis of silica-included HPM shows that the concentra-

ion of Si is 44.32 wt.% (stoichiometric value is 41.53), while the Oontent is 47.9 wt.% (stoichiometric value is 51.8).

For silica-included HPVM the concentration of Si is 39.7 wt.%stoichiometric value is 41.53), while the O content is 53.4 wt.%stoichiometric value is 51.42).

Fig. 10. (a and b) Microanalytical data of a 10 �m × 10 �m area and quantitativeresults of HPVM in SiO2.

It could be observed that both HPM and HPVM silica compos-ites exhibit a small deviation of Mo concentration values from thestoichiometric ones, probably owing to higher active phase con-centration supported at the surface of the silica–HPAs composites.

4. Conclusions

Molybdophosphoric acid H3[PMo12O40]·xH2O and 1-vanado-11-molybdophosphoric acid H4[PMo11VO40]·yH2O could beincluded in silica matrix by means of sol–gel technique to give aninsoluble and easily separable solid catalyst.

The intrinsic properties of the Keggin polyanion are still retainedafter immobilizing of HPM and HPVM into silica. According to IR andRaman spectra, the HPAs anions preserved their Keggin structureon the surface of silica–HPA composites. The X-ray powder patternsproved that HPAs are uniformly dispersed in the silica network.

Both silica–HPM and silica–HPVM composites exhibit differen-tial pore size distribution within mesopore and micropore range.

From the SEM micrographs it can be seen that silica–HPAscomposites are composed of spherical particles with an averagediameter of approximately 20–30 �m.

The favourable effect of HPAs incorporation on silica matrix isthe increase of pore volume and specific surface area, which in factmake the silica–HPA composites proper for heterogeneous cataly-sis.

Acknowledgments

These investigations were partially financed by the RoumanianMinistry of Education and Research, Grant CNCSIS No. 78 GR/2007-2008 and the Ministry of Science, Republic of Serbia – Project No.

142024 “To Green Chemistry via Catalysis”.

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